In a vehicle, an antilock brake system (ABS) loosens brakes when a tire locks while braking, and applies the brakes again when the tire begins to turn. The ABS operates so that a tire can exert the greatest frictional force. Therefore, when the vehicle is braked on a wet or frozen road where it tends to slip, there is an advantage that the braking distance becomes shorter and the steering control is maintained.
However when a large engine brake acts during antilock brake control, the same state is obtained as when the foot brakes are applied, and the recovery of the tires from the locked state is delayed even if the ABS loosens the brakes. When recovery of the tires from the locked state is delayed, the ABS does not exhibit its true performance.
Tokkai Hei 3-79851 published by the Japanese Patent Office in 1991 discloses that the effect of the engine brake is suppressed by reducing a drive ratio of the CVT transmission during the antilock brake control. Herein, the drive ratio corresponds to a gear ratio in the conventional transmission.